In the drilling of directional wellbores, conventional drilling methods of rotating a drill bit on the lower end of a pipe string is inadequate to create the curved portion of the borehole. Thus, mud motors, which include a bent section with a bend of curve of generally up to 3 degrees are used to drill the curved portions.
The conventionally used mud motors consist of three major components: a power section consisting of a rotor and a stator; a drive shaft; and a bearing assembly. The power section converts fluid pressure from the drilling fluid being pumped into rotational energy. The rotor is typically a helically fluted shaft that rotates eccentrically within the stator. The drive shaft must transfer the eccentric rotation and torque from the rotor to a concentric rotation and torque to the bearing assembly. The drive shaft must also transfer the thrust load from the rotor to the bearing assembly. The bend plane of the mud motor, used for directional drilling, generally lies within the drive shaft housing. Therefore, the drive shaft must also accommodate this bend. For these reasons, the drive shaft must be sufficiently robust to withstand the tremendous torque of the power section while having the ability to articulate in order to accommodate the eccentric rotation of the rotor and the bend in the drive shaft housing.
The vast majority of drive shafts in current mud motors fall within two main categories: constant velocity shafts and clutch type shafts. There are many variations of constant velocity shafts but all have the same characteristics are those used in automobile or All-Terrain Vehicle applications. In automobiles, for example, these constant velocity shafts are used in front wheel drive cars to connect the differential to the wheels to transfer torque, while accommodating varying angles of the suspension travel. These constant velocity shafts all use some type and number of balls or rollers to transfer torque while allowing some flexibility in their range of motion. These constant velocity shafts use many parts per assembly, including the torque transmitting items in addition to including sealing mechanisms. These sealing mechanisms often require multiple parts or special tooling for assembly. Most of these sealing mechanisms use some form of elastomeric seals which are simply ineffective in oil well down-hole conditions of high temperature and pressures. These constant velocity shafts are very expensive and difficult to maintain.
Clutch type shafts are far simpler and require far fewer parts. However, these shafts provide a very crude and rough articulation due to only having two lobes per mating component. This greatly reduces the “smoothness” of the motions, thereby reducing efficiency and increasing wear. The two lob design is prone to significant wear during use due to the “rubbing” of mating surfaces. Attempts have been made to place hard metals on abutting surfaces to prolong the longevity of these shafts, but this does not resolve the issue causing the wear.
Therefore, due to disadvantages of prior art such as complexity of design, excessive wear, insufficient strength to transmit the required torques, or non-uniform rotation, there is a need for an improved drive shaft capable of withstanding the torque of power sections without being susceptible to the negative effects of these torque demands, or from high temperatures, high pressures, and other factors associated with a mud motor in a wellbore drilling environment.